I. Field
The present invention generally relates to telecommunications, and more particularly, to providing fast and reliable change of communication cells to a user in a wireless communication system.
II. Background
In telecommunications, especially wireless communications, communication environments are not static but rather dynamic. In a mobile communication setting, some communication entities such as a User Equipment (UE) operated by a user may move from one location to another at different points in time.
Reference is directed to FIG. 1 which shows a simplified schematic illustrating an exemplary communication system. In the following description, terminology associated with the Wideband Code Division Multiple Access (WCDMA) or Universal Mobile Telecommunications Systems (UMTS) is used. The terminology and basic principles of operation of a UMTS communication system can be found from 3GPP (3rd Generation Partnership Project) Specifications 25.211-215, etc., published by the 3GPP.
In FIG. 1, there is a core network 20 linked to the Internet 22 and the Public Switched Telephone Network (PSTN) 24, for example. The core network 20 provides access of the Internet 22 and the PSTN 24 to subscriber users, such as a user operating a User Equipment (UE) 26, via a Universal Terrestrial Radio Access Network (UTRAN) 28.
Within the UTRAN 28, there is a Radio Network Controller (RNC) 30 linked to a plurality of cells, two of which are shown and designated by the reference numerals 32 and 34. Each of the cells 32 and 34 can be covered by one or different Node Bs (not shown). The Node Bs are terrestrial base stations capable of communicating with the UE 26 wirelessly. The cells 32 and 34 can be served by one Node B or separate Node Bs. If the cells 32 and 34 are served by one Node B, the cells 32 and 34 are sometimes called sectors of the serving Node B.
Suppose in FIG. 1, the UE 26 initially communicates with the cell 32. The cell 32 is called the serving cell for the UE 26. Even though the UE 26 is currently communicating with the cell 32, the UE 26 monitors and maintains the pilot signals from few other cells. Information of these other cells, called the “active set,” is stored in the memory of the UE 26. Suppose the UE 26 thereafter moves to the coverage area provided by the cell 34. The UE 26 senses the proximity with the cell 34 by receiving strong pilot signals from the cell 34, for instance.
With closer proximity and better signal strength, suppose the UE 26 decides to handoff the communication session from the cell 32 to the cell 34. To accomplish this end, the UE 26 needs to exchange messages with various entities. Heretofore, messages exchanged during handoff have mostly been designed to go through cells irrespective of the signal strength of signals received by the UE 26.
Reference is now returned to FIG. 1. The UE 26 starts the handoff process by sending a message with information regarding the pilot strength of all the cells in its active set to the RNC 30, via either the cell 32 and the cell 34, or both, as identified by the message paths 36 and 37, respectively, as shown in FIG. 1. As part of the message, the UE 26 may also report one particular cell has the strongest pilot signal and wish to switch to that cell as the serving cell.
Upon receipt of the message, the RNC 30 weighs the decision of whether to approve the handoff. The RNC 30 makes the decision based on a number of factors, such as the reported pilot strength and loading of the cells 32 and 34.
Suppose in this example, the RNC 30 approves of the serving cell change from the cell 32 to the cell 34. The RNC 30 sends a reconfiguration message which has parameters for accessing the cell 34 to the UE 26 via the cell 32. The path of the reconfiguration message is designated by the reference numeral 38 as shown in FIG. 1. The reason for sending the reconfiguration message through only the cell 32 is because the cell 32 is still the serving cell for the UE 26.
Suppose the UE 26 successfully receives the reconfiguration message via the cell 32. Based on the information of the reconfiguration message, the UE 26 can access the cell 34. If successful, the UE 26 sends a message to the RNC 30, again via the cells 32 and 34, in a manner similar to that as shown by the message paths 36 and 37, respectively, as previously described. The message basically reports the success of the handoff process.
The aforementioned serving cell change process can be successful if the communication conditions are favorable. However, in reality, communication conditions are not always favorable. Returning to FIG. 1, if the UE 26 is closer to the cell 34 and farther away from the cell 32, very possibly, the signal strength between the cell 32 and the UE 26 would be weak. Consequently, messages exchanged between the cell 32 and the UE 26, such as the messages sent via the paths 36 and 38 shown in FIG. 1, can be lost. This is especially true under certain scenarios. For instance, in an urban setting, changes of signal strength can be quite abrupt, which changes are mostly caused by dense city buildings. If the user of the UE 26 is in the midst of a Voice of IP (VoIP) call, inability to handoff the communication session from the cell 32 to the cell 34 can result in a dropped call.
Accordingly, there is a need to provide reliable and fast scheme for serving cell change in a wireless communication system.